Texture painting on backface
Often I find that view camera affects my painting as well. – Craig D Jones


It's my problem, I have disabled the "Norm" option  and now I can paint everywhere. This option is located in the left panel of 3D View in texture paint mode (show with T) > Option tab > Paint by projection > Normal

Uncheck it and  nowyou can paint on faces at 180° of the view...